Tucson riders unite at the Greater Arizona Bicycle Association Swap Meet.  Buy, sell and trade new and used riding gear at this biannual event held on Fourth Avenue between Sixth and Ninth streets.  The event is free and runs 6 a.m. to 1 p.m.  Parking meters and University of Arizona parking garages are free on Sundays.  For more information, call 624-5004 or visit www.fourthavenue.org.